The present invention relates to an optical semiconductor device and a method of producing the same and, more particularly, to an integrated semiconductor device feasible for high temperature or high optical output operation because of a minimum amount of current leak to occur from its current blocking layer, and a method of producing the same.
Today, the applicable range of optical communication is extending from trunk lines to branch lines or even to access lines. Semiconductor lasers for optical communication are therefore required to have advanced performance and new functions. For a trunk line, for example, a semiconductor laser whose drive spectrum involves a minimum of jitter at the time of modulation is essential in order to implement high speed, long distance transmission. A distributed feedback (DFB) laser having an electro absorption type optical modulator integrated therewith is one type of semiconductor laser meeting the above requirement. As for an access line, there is an increasing demand for a semiconductor laser to be easy to mount and capable of being efficiently coupled to an optical fiber. This kind of laser may be typified by a so-called spot size conversion type laser including a portion for converting the size of a beam.
Japanese Patent Laid-Open Publication No. 62-102583, for example, teaches a semiconductor laser desirable in high temperature or high optical output characteristic. However, the problem with the laser taught in this document is that the characteristic and yield are limited for the following reasons. To form layers different in band gap and thickness on a single substrate, crystal growth and crystal etching must be repeated, increasing the number of steps. Moreover, it is difficult to connect waveguides different in band gap and thickness accurately. As a result, a light loss occurs at the connecting portion and deteriorates the optical output characteristic.
To implement an integrated semiconductor laser, selective metalorganic vapor phase epitaxial growth (MOVPE) procedure capable of forming waveguide layers different in band gap and thickness collectively is attracting increasing attention. A semiconductor laser produce by selective MOVPE is disclosed in, e.g., the Papers of 56th Japanese Science Lecture Meeting, 1995, 27p-ZA-7, p. 930. This kind of scheme, however, brings about a problem that a current blocking layer included in the laser cannot function sufficiently in a high temperature environment or on the injection of a great current. Specifically, while a laser portion usually has the smallest band gap, it is necessary with selective MOVPE to increase the width of an anti-growth mask in the waveguide portion where the band gap should be reduced, i.e., the laser portion. It follows that in the laser portion the current blocking layer implemented only by InP and having a p-n-p-n thyristor structure has a broad area at both sides of an active layer. As a result, current leakage from the current blocking layer and ascribable to the storage of electrons is aggravated.